Robyn,I missed the part where you tell us about your dad,I am really sorry.
Sometimes a much loved pet is a reason for elderly owners to get up in the morning,a reason for them to pull them selves up and do their best.
I've seen it and although Slim might not get all the exersize he needs he is with the person he loves.
Your dad obviously knows you are wonderful pet-owners and when the time comes that he cannot cope with Slim,he can feel good about where he's going.